Foldables: Implement emulation for folded screen

Include ag/5696684 to fix an issue where the folded state was not
properly applied if a display override was restored after boot.

Bug: 123245311
Test: adb root && adb shell setprop persist.debug.force_foldable true && adb shell stop && adb shell start; covering the proximity sensor will half the available display area.
Test: Fold, power off, unfold, power on, verify entire display is usable.
Test: atest WmTests
Change-Id: I4d1eda8e7796bf30549c1154e3e041f9520bea24